1. Field of the Invention
The present invention relates to a method of surface treatment for reducing the abrasiveness of flexible tape-like material webs which are provided on at least one side with a coating which contains abrasive particles, the coated side of the material web being brought into frictional contact with the coated side of the deflected material web while running in the opposite direction thereto and particles present on the coated side of the material web subsequently being removed, and to an apparatus for carrying out the method for surface treatment.
2. Description of the Related Art
A number of apparatuses and/or methods are known in which the surface of flexible tape-like material webs, for example of magnetic recording media and in particular video tapes, are treated. The primary aims of this surface treatment are
removing small particles from the web surface to reduce the dropout rate PA1 reducing the magnetic head abrasion since particularly magnetic recording media may have particles projecting from the surface, for example magnetizable particles or nonmagnetic inorganic or organic particles, which have an abrasive effect, with the result that the magnetic heads for recording and playback of magnetic recordings rapidly become worn. This plays a particular role in duplication, when large amounts of magnetic recording media are provided with magnetic recordings at high speed.
In the TMD process in which, during contact copying with a master tape, a magnetic recording medium is heated to the Curie point of the magnetizable particles, it is also necessary for the abrasive behavior of the magnetic layer to be minimized, most materials of this type containing hard CrO.sub.2 pigments as magnetizable particles.
Methods and apparatuses for the surface treatment are disclosed, for example, in DE 28 03 914, DE 38 00 196, DE 38 12 851 and Japanese Applications 04/182929, 04/184712, 38115 (1991) and 02/285516 and U.S. Pat. No. 5,012,548. According to the abovementioned documents, either the layer surface of the material web is brought into contact with a special abrasive tape, which is moved at a defined contact pressure in the opposite direction to the tape, or the layer is passed over rotating cylindrical rollers having a defined roughness, or the layer is moved over a cylindrical roller which has hard edges and also rotates. A possible disadvantage of the abovementioned arrangements is that the surface of the abrasive tape or of the abrasive cylinders changes during continuous operation, so that the abrasive effect changes after a certain time and a uniform effect therefore cannot be achieved.
DE 38 00 197 discloses a method for the surface treatment of substrate sheets, in which the latter are faced and then finely processed with lapping wheels with the addition of a polishing liquid comprising an aqueous solution containing alumina powder and organic additives, the lapping wheels being provided with a covering of a resilient material.
Arrangements for reducing the abrasiveness of coated material webs, in which the layer side is passed over a sharp edge, are also known. Such arrangements are described in DE 21 19 094, DE 32 11 226, DE-A 42 20 880 and Japanese Application 03/288326. According to U.S. Pat. No. 3,616,478, a magnetic tape passes over a blade which consists of a crystalline ceramic material and which should make a certain angle with the magnetic tape. U.S. Pat. No. 3,674,579 describes a method in which the layer side of the tape to be cleaned passes over the parallel edges of a continuous steel belt which is clamped between two rollers, one of which is driven by a motor.
These apparatuses have the disadvantage that, in the case of a broad material tape, expensive measures have to be taken in order to remove particles adhering to the blades, for example by air extraction. This disadvantage can be partly overcome if the material strips which are respectively cut to the width for use are first passed over the blades. However, very expensive apparatus is then required. However, both cases have the fundamental disadvantage that, after a certain time, the blades have worn out and must be replaced.
Methods and apparatuses for removing small particles present on the layer, for example by extraction, wiping off or separation or by electrostatic means, are described in the publications U.S. Pat. No. 5,144,513, SU 1 675 942, U.S. Pat. Nos. 3,102,667, 5,077,881 and 4,887,623.
A method frequently used for the surface treatment of flexible tape-like material webs is calendering, in which coated plastic films are drawn through two rotating calender rolls which press against one another under defined pressure and at a defined temperature. Calendering is usually carried out in the longitudinal direction of the film web. Investigations by the Applicant have shown that the abrasiveness of the coating cannot be satisfactorily reduced by calendering alone.
A method of the generic type stated at the outset is described in U.S. Pat. No. 2,688,567, in which the coated sides of a magnetic recording medium are transported past one another in opposite directions with linear contact between two movable pressure rollers in order to be calendered in this manner. According to the statements in this publication, calendering is presumably achieved by pressing the abrasive particles into the plastically deformable layer. Furthermore, linear contact is scarcely likely to be suitable for reducing the abrasiveness in a defined manner.
It is an object of the present invention to provide a method for treating the surfaces of tape-like material webs which does not have the disadvantages of the abovementioned prior art, which can be carried out reproducibly and cheaply and in which the abrasiveness of the layer is markedly reduced.
We have found that this object is achieved by a method and apparatus for carrying out the method in which the abrasive coated surface is treated with itself by passing the tape in such a way that the surfaces come into frictional contact with one another over a given length.